In an effort to reduce the amount of pollution emissions from gas-powered turbines, governmental agencies have enacted numerous regulations requiring reductions in the amount of oxides of nitrogen (NOx) and carbon monoxide (CO). Lower combustion emissions can often be attributed to a more efficient combustion process, with specific regard to fuel injector location and mixing effectiveness.
Early combustion systems utilized diffusion type nozzles, where fuel is mixed with air external to the fuel nozzle by diffusion, proximate the flame zone. Diffusion type nozzles produce high emissions due to the fact that the fuel and air burn stoichiometrically at high temperature to maintain adequate combustor stability and low combustion dynamics.
An enhancement in combustion technology is the utilization of premixing, such that the fuel and air mix prior to combustion to form a homogeneous mixture that burns at a lower temperature than a diffusion type flame and produces lower NOx emissions. Premixing can occur either internal to the fuel nozzle or external thereto, as long as it is upstream of the combustion zone. An example of a combustion liner used in a premixing combustor of the prior art is shown in FIG. 1. The combustion liner 100 is generally cylindrical in shape and includes an inlet end 102 and an opposing outlet end 104. A plurality of fuel nozzles (not shown) are positioned at the inlet end 102 for injecting fuel into the combustion liner 100 to mix with compressed air and form a fuel-air premixture. This premixture is then ignited and burns to form the hot combustion gases used to drive then engine's turbine and provide either thrust for propulsive power or mechanical energy for driving an electrical generator.
Combustion systems for a gas turbine engine can come in a variety of configurations. Generally, for the purpose of discussion, the gas turbine engine may include low emission combustors such as those disclosed herein and may be arranged in a can-annular configuration about the gas turbine engine. One type of gas turbine engine (e.g., heavy duty gas turbine engines) may be typically provided with, but not limited to, six to eighteen individual combustors, each of the combustors having a casing, flow sleeve, combustion liner, fuel nozzles and an end cover.
In order to regulate the amount of air provided to a low emissions combustor, a flow sleeve 110 is positioned around the combustion liner 100, as shown in partial cross section in FIG. 2. More specifically, the combustion liner 100 is located radially within a flow sleeve 110 while the flow sleeve 110 is located radially within a combustor casing 112. The flow sleeve 110 is secured in an axial position within the casing 112 by a forward flange 114 which is seated within a casing flange 116. However, like the flow sleeve 110, the combustion liner 100 also must be secured in the proper axial position such that the fuel nozzles and other mating hardware (not shown) are properly positioned within the gas turbine combustor.
Combustion systems of the prior art have attempted to position and secure the combustion liner 100 in place in a variety of manners. For example, with reference to FIGS. 1-3, a common technique of securing a combustion liner in a flow sleeve is for the liner to include a plurality of T-shaped tabs 118 which extend radially outward from the combustion liner 100 and are received within slots of corresponding flow sleeve pegs 120. However, as it can be seen in FIG. 3, this technique includes a thick T-shaped liner tab 118 secured to the outer wall of the combustion liner 100 and such a thick tab design results in high thermal and mechanical stresses due its geometry, the amount of load on each tab and welding techniques applied for the joint between the tab and the combustion liner. A similar T-shaped style liner tab 118 is shown on the outer surface of an alternate combustion liner in FIG. 4.
An alternate prior art design for securing a combustion liner in a flow sleeve is depicted in FIGS. 5 and 6. In FIG. 5, a combustion liner 500 of the prior art is shown in perspective view and includes four equally-spaced liner tabs 502. In this combustion system, due to the configuration of the fuel-air mixing and combustion, the liner tabs 502 were subjected to high operating temperatures and high thermal stresses since the liner tabs 502 were located radially outward of the combustion zone. The liner tabs 502 are shown in greater detail in FIG. 6. The liner tab 502 has a general pitch-fork like shape to it, including an inverted generally U-shaped portion 504 and tab extension 506. The generally U-shaped portion 504 is welded to the combustion liner 500 along joints 508. This prior art design attempted to remove the thick liner tab portions from the combustion liner, as discussed above with respect to FIGS. 1-4. However, despite these design changes, the joints 508 experienced some of the highest operating stresses in the combustion liner 500. Such high stresses were due at least in part to the stiffness of the liner tabs 502.
Therefore, it is necessary to identify a design alternative that meets the mechanical and thermal loading conditions of the combustion liner and flow sleeve interface such that the combustion liner can be secured in its proper position and not be subjected to failure.